1. Field of the Invention
This invention pertains to communication switching systems, particularly to those for interconnecting a plurality of individual stations both with the local and satellite-linked other stations.
2. Background of the Invention
The prior art most directly related to the present invention is the inventor's U.S. Pat. No. 3,842,350, which is assigned to the assignee of the present application, and is included herein by reference. That patent teaches the provision, for each individual subscriber line or trunk, of a trunk access unit, abbreviated to TAU, which has frequency conversion means to convert incoming signals (from subscriber) to an intermediate frequency which is determined by the frequency of a programmable local oscillator, whose frequency is determined by digital control signals from a routing controller. Cognate equipment in the TAU has a similar programmable local oscillator whose frequency determines the received intermediate frequency which will be demodulated to an outgoing signal to subscriber. The intermediate frequencies used cover a band which may be less than the total band used for communication with the satellite. Therefore a number of up-converters may be provided to receive intermediate-frequency signals and convert them to a frequency lying in a particular part of the total band used to transmit to the satellite. Each TAU is provided with a switching system to connect its intermediate-frequency output to a selected up-converter. Similarly, a number of down-converters may be provided to select a particular part of the total band used to receive from the satellite; the TAU has a second switching system to connect its intermediate-frequency input to a selected down-converter. In the referenced patent one channel in each of the TAU switching systems is connected to a bus-type channel common to all of the local TAUs, so that the interconnection of two local TAUs requires a switching operation in addition to appropriate intermediate-frequency assignment. In the patented system, it is contemplated that the same intermediate-frequency band will be used both for communication via the satellite with remote TAUs, and for communication between local TAUs. The assignment of intermediate frequencies and, for satellite-relayed calls, of the particular up-converter and down-converter to be used, is controlled locally by a data-processing device known as a routing controller, which may be directed by control signals from a system routing center located at one ground station in a system of ground stations. Information on the frequency assignments currently in actual use is maintained by the data-processing equipment and unused frequencies are assigned at need. Thus there is no frequency particularly assigned to a given TAU, except for the duration of a given call. Since only a fraction of all TAUs will ordinarily be in use at any given time, this permits fuller utilization of the available frequency spectrum.
U.S. Pat. Nos. 3,809,815 and 3,809,816, of Reed et al. teach a system in which a plurality of telephones are connected to a common broadband channel, such as a coaxial cable, with a transmitting or receiving frequency permanently assigned to each telephone. Connection between two telephones is made by suitably adjusting the receiving or transmitting frequency of one to correspond to the receiving or transmitting frequency of the other telephone. There is no suggestion of interconnection via a satellite or other relay, or of the use of up or down converters to cover a band broader than the band of intermediate frequencies employed.
U.S. Pat. No. 3,065,420, of Close, discloses a transmission system operating at intermediate, or converted, frequencies in which reliability is improved by an unusual form of redundancy. Two different information signals are both fed to two different modulators. Each modulator is connected to a different demodulator, and in normal operation one signal is selected from each demodulator. However, the transmission channels for the intermediate frequencies are cross connected so that, in an emergency caused by failure of either modulator, or either demodulator, both signals will be transmitted, and both may be demodulated, from a single demodulator if need be. This is completely different in use, concept, and teaching from the present invention.
No prior art is known to the applicant in which signals at intermediate frequency are directed to local common channels or to satellite relay channels by frequency selectivity of permanently connected ties to the two channels.